1. Field of Invention
The present invention relates to an antenna apparatus. More particularly, the present invention relates to method and apparatus for improving antenna radiation patterns.
2. Description of Related Art
Mobile telephones are portable and wireless telephone devices installed on conveyances, such as vehicles and ships, or carried by a user. Mobile telephones are different from wireless extensions of the wired telephones or long distance radio transceivers. Mobile telephones provide users with the benefits of the same functions of and greater convenience than wired telephones. Connecting with international direct dialing, mobile telephone users can communicate with any other person in the world within coverage of a mobile telephone system.
A mobile telephone network system comprises mobile telephone switching offices (MTSO), base stations (BS) and mobile stations (MS). Generally, the mobile telephone network system may have one or more than one MTSO's, which comprise switches and communication devices, and govern a certain number of base stations. The communication devices of the MTSO are connected to the base stations.
A cellular wireless network is composed of several cells, and every cell has its own transmitting/receiving module (TRM), control channels (CC) and communication channels. The base station services one or more cells according to coverage requirements and with different antenna designs. The quantity, coverage regions, and frequency bands of the channels can be selected according to the service requirement of the cellular wireless network.
The mobile station is the mobile telephone mentioned above, which is a radio transceiver and a control unit for sending signals to the base station. When the mobile station intents to communicate with any other person, signals are transmitted to the MTSO via a wireless channel of the base station assigned to the mobile station, and then are forwarded to a public switched telephone network (PSTN). Every mobile station can receive and make a call from a subscribed MTSO, and also can make a call from other MTSO's by roaming. Besides, when a mobile station in use moves from one cell to another, the channel of the mobile station is automatically switched to that of the new cell.
From the above descriptions, when the base station services a larger quantity of mobile stations in a region such as a downtown area with a large population, the base station must have a larger capacity for dealing simultaneously with the higher communication load from many mobile stations. The mobile telephone network system applies a frequency reuse technique to increase the system capacity, and thus a large quantity of the base stations must be deployed.
Generally, the antenna used in the base station is a directional antenna, called a sector antenna, and the advantage of the antenna is that the energy of the antenna can be concentrated on a sector region. For the mobile telephone network system, the directional antenna is very helpful to the deployment of the base stations. However, in practice, the antenna radiation pattern of the sector antenna has an unwanted back lobe that radiates energy backward to other cells.
Since a cellular system adopts the above-mentioned frequency reuse technique, two signals of the same channel arriving at a mobile station from different base stations will interfere with each other. Thus, a larger back lobe of the base-station antenna radiation pattern will cause more interference to the service regions of other base stations. Also, the base-station antennas are usually mounted on top of buildings. Therefore, the back lobe radiations will more likely cause co-channel interferences to adjacent base stations assigned with the same frequencies.
In the prior art, in order to prevent mobile stations from co-channel interferences between different base stations and the communication quality thereof from being affected, the usual solution is to increase the distances between base stations using the same channels. However, this solution reduces the quantity of base stations in a certain area, and decreases the signal strengths over some regions in the certain area. Also, if the base-station antenna is mounted on top of a building, a conventional approach for reducing the co-channel interferences is to place a metal-grid panel behind the base-station antenna to shield the back-lobe radiation. However, this conventional method would affect the outer appearance of the base station, increase the wind resistance of the base-station antenna, require higher cost, need more construction efforts, and yet only provide smaller improvement.